1. Technical Field
The present invention relates to a position detecting method, an information recording/reproducing device and an information recording medium which are used for detecting a head position according to a servo pattern and locating the head at the time of recording/reproducing. Particularly the invention relates to the position detecting method, the information recording/reproducing device and the information recording medium which are used for detecting the head position accurately even if recording shift occurs in the servo pattern.
2. Description of the Related Arts
At the present, in a head locating control method which is called as sector servo used in a magnetic disc device, servo areas where servo information is recorded are radially arranged on a disc with an equal angle. A head is located on a target track based on the servo information. The servo information is composed of track addresses and a servo pattern (position information signal pattern). The track addresses are used for identifying respective tracks. The servo pattern is used for obtaining a quantity of offset from a center of the track of the head.
FIG. 1A illustrates one example of the prior servo pattern, and black-out portions are supposed to be called as pattern elements. The area of the servo pattern is magnetized to a circumferential direction, but a magnetic field of the portions of the pattern elements directs to the opposite direction. The pattern elements whose radial width is about 1 track are arranged with intervals of about 1 track, namely, the pattern elements are arranged with a period of 2 tracks so as to form one burst area. The servo pattern is composted of four burst areas 102A, 102B, 102C and 102D which are shifted by ½ track in a radial direction. When a head 104 is in a position of FIG. 1A, a produced signal of the head in FIG. 1B is obtained. An amplitude of the reproduced signal changes according to a percentage that the head 104 enters the pattern elements. When the head 104 is in a vicinity of a cylinder boundary as shown in the drawing, a difference in the amplitudes of reproduced signals 106A and 106B from the burst areas 102A and 102B is determined as the head position. At this time, reproduced signals 106C and 106D from the burst areas 102C and 102D are zero or in an area where their amplitudes are saturated, and their linearity with respect to the head position is lost. For this reason, the reproduced signals 106C and 106D are not used. When the head is in a vicinity of a cylinder center, a difference in the amplitudes of the reproduced signals 106C and 106D from the burst areas 102C and 102D is determined as the head position. At this time, the reproduced signals 106A and 106D from the burst areas 102A and 102B are not used. A method of detecting the head position in such a manner is called as an amplitude demodulating method.
FIGS. 2A and 2B illustrate another example of the prior servo pattern, and this is called as a phase demodulating method. In the phase demodulating method, a portion, where a plurality of parallel lines are drawn so that phase differences in the circumferential direction and the radial direction is proportional to each other, is one burst area 108A, and a portion where the burst area 108A is inverted axisymmetrically in the circumferential direction, is another burst area 108B as shown in FIG. 2A. In the phase demodulating method, when head positions 110-1 to 110-5 are taken for example, a phase difference between reproduced signals 112A and 112B from the two burst areas 108A and 108B is directly the position of the head in the radial direction.
When the servo pattern is recorded into the disc accurately without a shift, the head position is detected accurately. When the servo pattern is shifted, however, an error occurs in a head position signal to be detected. In the case of the amplitude demodulating method, a shift of the servo pattern in the radial direction appears directly as an error of the head position signal. If the servo pattern is deviated in the circumferential direction, however, the amplitude of the reproduced signals does not change that much, and thus the error does not depend on the shift in the circumferential direction that much. Dispersion of the error in the position signal is approximately inversely proportional to a total sum of circumferential parallel sides of the pattern elements present in an area where the head passes. For this reason, as a number of the pattern elements where the head passes is larger, the condition is better, but only two of the four burst areas can be used for detection of the position signal. For this reason, the usability of the position information signal pattern is not good, and thus this method is wasteful. In the case of the phase demodulating method, the shift of the servo pattern in the radial direction is directly an error of the position signal of the head. Further, the shift in the circumferential direction is also a phase shift of a reproduced signal, and thus becomes an error of the position signal. When a period in the circumferential direction is Px and a period in the radial direction is Py, the shift in the circumferential direction is multiplied by (Py/Px) so as to appear as the error of the position signal. When an inclination is set down so that the lines face the circumferential direction, (Py/Px) becomes small, but as a result, the amplitude of the reproduced signal becomes small, and the error of the position signal is enlarged due to deterioration of SN ratio. For this reason, it is necessary to maintain the inclination to a certain extent or more. The dispersion of the error of the position signal is approximately inversely proportional to a total sum of side lengths of the pattern present in the area where the head passes. Since the signals from the two burst areas are always used, the usability of the position information signal pattern is not waste. The amplitude demodulating method has a problem that the usability of the servo pattern is not good, whereas the phase demodulating method has a problem that the shift of the pattern in the circumferential direction influences accuracy of the position signal.